SELECTIVE PARTICLE INGESTION BY OYSTER LARVAE (CRASSOSTREA-VIRGINICA)FEEDING ON NATURAL SESTON AND CULTURED ALGAE

I investigated selective particle ingestion by oyster larvae (Crassost rea virginica) feeding on natural seston from Chesapeake Bay and labor atory-cultured algae of different sizes or chemical content. In 15 of 16 experiments with complex natural suspensions as food, small (<150 m u m) and large (>150 mu m) larvae selected most strongly for small (2 to 4 mu m) food particles, but in the presence of a large (>10 mu m)-c ell dinoflagellate bloom, large larvae strongly selected much larger ( 22 to 30 mu m) food material (presumably dinoflagellates). When fed si mplified mixtures of four cultured algal species (Synechococcus bacill aris, Isochrysis sp., Dunaliella tertiolecta, and Prorocentrum minimum ) ranging in size from 1 to 11 mu m, small larvae preferred 1 mu m alg ae while large larvae preferred 11 mu m algae. In experiments with alg al mixtures, and with suspensions of natural particles and added algae , large larvae preferred algal species harvested from exponential-phas e cultures over other species from stationary-phase cultures. Larval i ngestion rates of the cultured alga Thalassiosira pseudonana were abou t three times higher for cells with a low carbon:nitrogen ratio (7.2:1 ) than for high C:N ratio (16.2:1) cells when these cells were offered separately in suspensions of equal concentration. As a result, more a lgal cells, algal C, and algal N was ingested by larvae fed low C:N ce lls. However, larvae did not show a significant preference for either type of cell when they were offered in a 1:1 cell mixture. Feeding pat terns of C. virginica larvae in natural food suspensions can vary with the composition of these complex suspensions, and ingestion seems dep endent not only on the size, but on the growth rate and chemical quali ty of food particles.